1. Field of the Invention
The present invention relates to a process for preparing a lower alkyl polysilsesquioxane adapted for forming an insulating layer of a silylated lower alkyl polysilsesquioxane on an electronic circuit board and a process for forming such an insulating layer which flattens stepwise differences in height due to micropatterns of conductors on an electronic circuit board.
2. Description of the Related Art
Insulating layers are commonly formed on electronic circuit boards having electronic elements, particularly integrated circuits, large scale integrated circuits, and magnetic bubble memory devices. The surface of these circuit boards exhibit stepwise differences in height due to the micropatterns formed thereon. Therefore, the insulating layers must exhibit flat step coverage. In addition, they must exhibit a thermal resistance to prevent thermal decomposition, a coefficient of expansion similar to that of the underlying material to prevent cracks, and a dense structure to prevent penetration of external gases or moisture, which would lead to deterioration of the performance of the electronic elements.
Insulating layers are usually made of inorganic oxides, e.g., phosphosilicate glass (PSG) and/or organic resins. Inorganic layers have the advantage of a relatively dense structure, however, in certain cases display insufficient step coverage. In addition, such layers produced by plasma chemical vapor deposition or sputtering have the disadvantage of not forming a desirable thickness in an acceptable duration of operation.
On the other hand, organic resins, e.g., polyimide, exhibit fair step coverage and, consequently, a flat surface profile. However, they have the demerits of a coarse structure, consequently, high hygroscopic property, lack of resistance against thermal decomposition and thermal shock. Furthermore, organic resins have to be readily dissolvable in an organic solvent to allow them to be applied to the electronic circuit board.
An organosilicone resin has been noted to exhibit combined features of organic and inorganic materials. Particularly, organopolysilsesquioxanes having a so-called ladder structure, i.e., a two-dimensional structure, are more resistant against thermal decomposition than organopolysiloxanes having a linear structure.
Brown et al report in U.S. Pat. No. 3,017,386 that phenyl polysilsesquioxane exhibits fair resistance against thermal decomposition.
It is known that phenyl polysilsesquioxane suffers from cracking in the process of curing, and that methyl polysilsesquioxane exhibits reduced cracking. Takiguchi et al, however, report in Japanese Unexamined Patent Publication (Kokai) No. 50-111198 that methyl polysilsesquioxane produced by a process in which methyl trichlorosilane reacts mildly with water has the demerit of insolubility in an organic solvent. This insolubility derives from the formation of a three-dimensional structure due to cross-linking at hydroxide groups, which are produced by hydrolyzation of methyl groups and not spent for polycondensation of polysilsesquioxane. Therefore, it is essential to conduct hydrolyzation of methyl trichlorosilane in extraordinarily mild conditions.
Brown et al to General Electric Co. disclose in U.S. Pat. No. 3,017,386 a process for preparing an organopolysilsesquioxane. A phenyl prepolymer is formed by reacting phenyl trichlorosilane in an organic solvent with alkaline water at a temperature of about 75.degree. C. to 175.degree. C., washing the hydrolyzate with water, and removing essentially all of the water by azeotropic distillation. The resulting solution of the prepolymer is then treated at a temperature of about 250.degree. C. to obtain an organopolysilsesquioxane having a polymerization degree of several hundred to several thousand or more.
Takiguchi et al to Shin-etsu Kagaku Kogyo Ltd. disclose in Japanese Unexamined Patent Publication No. 50-111198 a process for preparing a lower alkyl polysilsesquioxane. First, a prepolymer of a number average molecular weight of 9,000 to 10,000 is formed by dissolving methyl trichlorosilane together with triethyl amine in a ketone-furan solvent and hydrolyzing ice-cooled methyl trichlorosilane with dropping water under normal pressure. Second, the prepolymer is precipitated by adding methyl alcohol to the hydrolyzed solution and then, dissolved again in a ketone-furan solvent and heated at 90.degree. C. for 4 hours to obtain a polymer of a number average molecular weight of about 10,000 to about 100,000.
Sumie et al to Nihon Gosei Gomu Ltd. disclose in Japanese Unexamined Patent Publication No. 53-88099 a process similar to that of Takiguchi et al, Sumie et al found that the obtained methyl polysilsesquioxane became insoluble after further standing at room temperature, although the polymer has a relatively low number average molecular weight of about 5,000. Sumie et al treated the once separated prepolymer with an ammonium salt dissolved in water or methyl alcohol at about 90.degree. C. for 4 hours to obtain methyl polysilsesquioxane of a number average molecular weight of about 20,000 to about 100,000. They found that the obtained polymer did not become insoluble after standing.
In regard to organopolysilsesquioxanes adapted for use as an insulating layer, as we previously stated, phenyl polysilsesquioxane exhibits a fair resistance against thermal decomposition but not against thermal cracking and that a low alkyl polysilsesquioxane exhibits a resistance reverse to that of phenyl polysilsesquioxane.
Kamiya et al disclose in Yogyo Kyokai Shi, Vol. 92, No. 5, pages 242 to 247, 1984 a process to improve the thermal resistance of a diorganosiloxane polymer by replacing the active hydroxide end radicals with trimethyl silyl groups. However, such a silylated organopolysilsesquioxane has not been applied for insulating layers of an electronic circuit board.
Sudo et al to Hitachi Ltd. disclose in Japanese Unexamined Patent Publication No. 55-50645 an organopolysilsesquioxane layer formed on and near the surface of a semiconductor device. Most of the organic groups are selected from the groups of phenyl and methyl. Part of the organic groups are selected from the groups of C2 to C6 alkyl, vinyl, halogenized phenyl, hydroxyl alkyl, mercaptoalkyl, alkyl phenyl, and aminophenyl. Shoji et al to Hitachi Ltd. disclose in Japanese Unexamined Patent Publication No. 55-145355 a layer insulating layer of organopolysilsesquioxane in which organic radicals are selected from the groups of methyl, ethyl, propyl, butyl, phenyl, halogenophenyl, halogenomethyl phenyl, and amyl hexyl phenyl ethyl.
Tokunaga et al to Fujitsu Ltd. disclose in Japanese Unexamined Patent Publication No. 56-125857 a process for producing a semiconductor device in which methyl polysilsesquioxane is used to flatten the stepwise lower portion between underlying micropatterns of conductors and an inorganic insulating layer is formed thereon. However, Tokunaga et al do not refer at all to the active end groups of the methyl polysilsesquioxane.
Neither Sudo et al, Shoji et al, nor Tokunaga et al teach or suggest to replace the active end radicals of the organopolysilsesquioxane with trialkyl silyl groups.